The present invention is related to the field of telephone systems, and more particularly to automated call progress monitoring based on analog signaling information utilized in telephone systems.
Many modern telephone systems employ digital out-of-band signaling among system components in setting up, monitoring, and terminating connections for a telephone call. Although analog signaling tones such as ringing tone, dial tone, etc. are still utilized in such modern systems, they are used only at the edges of the network for the benefit of the participants in a call. Within the network, signaling information is passed in the form of digital messages that are processed by digital processing circuitry at the various system components. In the Signaling System 7 network, for example, a serving switch can send a data packet describing the real-time call progress. A serving SS7 switch can send ALERTING, BUSY, or INVALID messages to a calling SS7 switch. Likewise, in ISDN networks or networks employing Q. 931 signaling, a private branch exchange (PBX) can signal the network regarding the appropriate call progress state for a given call. The use of digital processors and signaling provides numerous benefits, including the ability to provide a variety of feature-rich services.
However, digital signaling is not yet ubiquitous in the global public switched telephony network. Even in the United States, there are still analog end office switches such as the AT&T/Lucent 1AESS, as well as analog PBXs, which have no provision for digital call progress signaling. In many other countries, analog switching and signaling equipment is still more the rule than the exception. Additionally, digital signaling capability within the network may be selectively disabled or otherwise unavailable for legal or contractual reasons. Even if a terminating service provider is capable of deploying equipment that can perform call progress analysis, it may choose not to. When digital signaling is unavailable, an originating service provider would like to perform call progress analysis to be able to provide accurate bills to its subscribers.
At present, when a call is placed to an analog far end switch, the network opens a “bearer channel” from the caller to the switch, and the far end switch generates analog call progress tones in the bearer channel. Examples of the signals being conveyed by these analog tones include ringback, busy, and special information tone (SIT). As an example of the format of these signals, the ringback signal in the U.S. takes the form of a combined 350 Hz tone and 440 Hz tone, with a cadence of 2 seconds on and 4 seconds off. The signal tones are interpreted by the caller, and in some cases may also be detected by electronic circuitry coupled to the channel. Once the called party answers, the serving switch issues answer supervision signaling, which is the start of billing in the U.S.
The above approach to monitoring the call progress in the absence of digital signaling has a serious drawback, which is the potential for toll fraud. A PBX can be programmed to never issue answer supervision. During the initial part of a call, the path between the caller and the called party is opened for the purpose of conveying the initial analog “media”, which includes analog signaling tones as well as the initial utterances of a called party when he or she answers the phone. This media connection is initially free, and only becomes subject to charges after the telephone system deduces that the call has been answered (i.e., answer supervision has been issued and detected). If answer supervision is never issued or detected, this path can be used for the entire call without either party incurring charges. From the point of view of the caller and called party, they have a regular telephone call. However, in the absence of answer supervision, the service provider is unaware that a billable call has occurred.
There are two approaches to solving this problem. The first is to limit the media path or the duration of the free initial portion of the call, either by disconnecting after a certain period of time or by charging for a call as if answer supervision had been issued after a certain period of time. This approach is in widespread use. The second approach is to employ circuitry to monitor call progress by examining the far-end media stream, and to utilize the information gleaned from such monitoring to detect when a call has been answered, as well as other important call progress states. The output of such monitoring circuitry can serve in place of far-end answer supervision, reducing the possibility of toll fraud.
The monitoring of call progress by using electronic circuitry to analyze analog signaling tones is known in the art, and is shown for example in U.S. Pat. Nos. 5,023,906, 5,325,425 and 5,521,967 of Novas et al. Generally, any such circuitry must be capable of recognizing the presence or absence of certain analog tones, which may appear in combination. Additionally, the circuitry must be able to recognize tone duration and cadence, and further must have some awareness of the stage of each call being analyzed. This need can be appreciated by considering the use of dial tone. At the beginning of a call, the presence of a dial tone is necessary for dialing to proceed, whereas the presence of dial tone later in the call likely indicates that the call has terminated or otherwise failed. Call progress analysis circuitry must be capable of distinguishing between these and other cases.
There can be drawbacks to the use of call progress analysis such as described above. In the system of Novas et al., for example, the call progress analysis circuitry is part of specialized customer premises calling equipment. Although the cost and complexity of this specialized equipment may be justified for some customers having corresponding special needs, this is not true for many customers. Additionally, the Novas et al. patents do not address the use of call progress analysis in the context of global calling, in which different analog signaling plans are utilized by far-end equipment residing in different countries. The Novas et al. patents describe operation based on an assumed signaling plan, such as the signaling plan used in the United States. An analysis process based on the U.S. signaling plan would be problematic if used on a call placed to Japan, for example. Japanese ring-back sounds similar to U.S. busy tone, and therefore the system of Novas et al. might mistakenly conclude that the line to a called party in Japan is busy when in fact it is ringing.
What is needed is a method of performing call progress analysis on a media stream that operates properly in conjunction with different call progress signaling plans, and that can be deployed for use by a variety of telephone customers in a cost-effective manner.